The present invention is directed to a method of and an apparatus for improving vision and the resolution of retinal images. More particularly, the present invention is directed to a method of and an apparatus for measuring and correcting the wave aberration of the eye such that the measured data can be used to develop corrective optical elements for improving the optical quality of the eye.
Despite significant advances in spectacle and contact lens design, current ophthalmic lenses still can only correct defocus and astigmatism. Spectacles and contact lenses leave uncorrected additional aberrations such as spherical aberration, coma, and a host of irregular aberrations. These high order aberrations of the eye not only blur images formed on the retina, which impairs vision, but also blur images taken of the living human retina. There have been two obstacles that prevent the use of specially-designed optical elements to correct aberrations beyond defocus and astigmatism in the eye. First, quantitative measurement of the irregular aberrations of the eye has not been possible. Second, a mechanism to correct the monochromatic aberrations of the eye other than defocus and astigmatism has not been demonstrated.
Subjective refractive methods of optometrists and objective autorefractors measure defocus and astigmatism only. They cannot measure the complete wave aberration of the eye, which includes all aberrations left uncorrected by conventional spectacles. The objective aberroscope disclosed by Walsh et al. in the Journal of the Optical Society of America A, Vol. 1, pp. 987-992 (1984) provides simultaneous wave aberration measurements of the entire pupil but cannot sample the pupil with a spacing finer than about 0.9 mm (See Charman in Optometry and Vision Science, Vol. 68, pp. 574-583 (1991)). Moreover, rapid, automated computation of the wave aberration has not been demonstrated with this method.
Recently, one of the co-inventors herein, together with others, developed an apparatus to measure the wave aberration of the eye. In a report entitled xe2x80x9cObjective measurement of wave aberrations of the human eye with the use of a Hartmann-Shack wave-front sensorxe2x80x9d, Liang et al., J. Opt. Soc. Am. A., volume 11, number 7, pp. 1-9, July 1994, the disclosure of which is incorporated by reference herein, the authors disclosed a Hartmann-Shack wavefront sensor that they used to measure the wave aberrations of the human eye by sensing the wavefront emerging from the eye produced by the retinal reflection of a focused light beam on the fovea. Using the system disclosed therein, the authors were able to measure only up to fourth order polynomial functions. The wavefront fitting with polynomials up to fourth order does not provide a complete description of the eye""s aberrations. That description is generally insufficient to accurately compute the optical performance of the eye. This instrument was not equipped to remove unwanted light reflected from other surfaces, such as lenses and the cornea of the eye.
There has also been a previous attempt to correct the monochromatic aberrations of the eye beyond defocus and astigmatism, with the goal of improving the axial resolution of the confocal scanning laser ophthalmoscope. Bartsch et al., in Vision Science and its Applications, 1994, Technical Digest Series, Vol. 2 (Optical Society of America, Wash., D.C.) pp. 134-137 (1994) used a fundus contact lens to null the refraction at the first surface of the cornea. That approach, however, suffers from the fundamental problem that the wave aberration of the eye depends on the combined effects of refractive index variations throughout the eye""s optics. Possibly for that reason, an attempt to use a fundus contact lens to increase the axial resolution of a confocal scanning laser ophthalmoscope showed only modest improvement.
Another approach is to use a deformable mirror, a device that has successfully compensated for atmospheric turbulence in ground-based telescopes. A deformable mirror was previously proposed for use in a confocal laser scanning ophthalmoscope in conjunction with the human eye in U.S. Pat. No. 4,838,679 to Bille, but no method to measure the wave aberration of the eye was proposed or disclosed. Dreher, Bille, and Weinreb, in Applied Optics, Vol. 28, pp. 804-808 demonstrated the only usage of a deformable mirror for the eye, but only corrected the astigmatism of the eye, which is no better than the correction provided by conventional ophthalmic lenses. The use of an optical element to correct monochromatic aberrations higher than second order has never been achieved. In both those systems, no appropriate method for measuring the eye""s high order aberrations was disclosed. Bille et al., in Noninvasive Diagnostic Techniques in Ophthalmology, edited by Masters, B. R., Springer-Verlag, pp. 528-547 (1990) proposed the use of a wavefront sensor in conjunction with a deformable mirror, but a working system was never disclosed or realized.
In view of the foregoing, it is apparent that there exists a need in the art for a method of and an apparatus for producing ophthalmic optical elements that provide improved or supernormal vision over that which is currently available, as well as high resolution retinal images. It is, therefore, a primary object of the present invention to provide a method of and an apparatus for accurately measuring higher order aberrations of the eye and for using the data thus measured to compensate for those aberrations with a customized optical element.
It is also an object of the present invention to provide an improved wavefront sensor which rejects light reflected from structures other than the retina and which is capable of providing a complete measurement of the eye""s aberrations.
It is a further object of the present invention to utilize such an improved wavefront sensor in combination with a deformable mirror to correct the wave aberration in a feedback manner such that the subject achieves normal or supernormal vision.
It is likewise a primary object of the present invention to provide a method of and an apparatus for producing high resolution retinal images which allow the imaging of microscopic structures the size of single cells in a human retina.
Briefly described, these and other objects of the invention are accomplished by providing a system for receiving light reflected from a retina of an eye. The wavefront in the plane of the pupil is recreated in the plane of a lenslet array of a Hartmann-Shack wavefront sensor. Each of the lenslets in the lenslet array is used to form an aerial image of the retinal point source on a CCD camera located adjacent to the lenslet array. The wave aberration of the eye, in the form of a point source produced on the retina by a laser beam, displaces each spot by an amount proportional to the local slope of the wavefront at each of the lenslets. The output from the digital CCD camera is sent to a computer which then calculates the wave aberration and provides a signal to a deformable mirror. Following an iterative procedure, the deformable mirror ultimately acquires a shape that is identical to the wave aberration measured at the outset, but with half the amplitude. This deformation is the appropriate one to flatten the distorted wavefront into a plane wave, which improves image quality.
In its method aspects, the system of the present invention, using the computer, first acquires the CCD image, as described above. Then, the computer computes the centroid of the spot formed by each of the lenslets of the wavefront sensor. Shifts in each focus spot in the x and y directions are calculated and then used as the slope data to fit with the sum of the first derivatives of 65 Zernike polynomials, using a least squares procedure, to determine the weight for each polynomial.
Then, the Zernike polynomials are weighted with the calculated coefficients. The 65 polynomials in the wavefront fit include all Zernike modes with radial power less than or equal to 10, except for the piston term.
The weighted Zernike polynomials are then added together, to result in the reconstructed wave aberration. The wave aberration is then evaluated at the locations of the actuators of a deformable mirror in order to produce the correction signal which is sent by the computer to the wavefront compensation device or deformable mirror as discussed above. Such a feedback loop continues to receive the reconstructed wave aberration results, feeding back an appropriate correction signal, until the RMS of the reconstructed wave aberration signal reaches an asymptotic value, at which point, the deformable mirror has been deformed such that it will compensate for all the detected aberrations of the eye.
When the reconstructed wave aberration signal reaches its asymptotic value, the final aberration signal (including all of the previously generated signals up until the RMS error reaches the asymptotic value) can be used to produce contact lenses to correct for all of the monochromatic aberrations of the human eye or for surgical procedures.
The present invention can also be used to provide high resolution images of the retina. The system for producing such images uses a krypton flash lamp which is designed to illuminate a retinal disk to provide an image of the retina which is reflected by the deformable mirror onto a lens and through an aperture such that the reflected image of the retina is focused onto a second CCD camera. The signals generated by the camera are acquired in a manner similar to that described above in connection with the first CCD camera and are stored for later use in the computer.
With these and other objects, advantages, and features of the invention that may become hereinafter apparent, the nature of the invention may be more clearly understood by reference to the following detailed description of the invention, the appended claims and to the drawings attached herein.